THE FIELD OF THE INVENTION
This invention relates to sealing or welding of plastic sheets or films. Radio Frequency voltage is applied to metallic highly conductive electrodes placed on opposite outer faces of two or more suitable juxtaposed plastic films. The Radio Frequency field created within the plastic tends to electronically excite those molecules within the plastic that demonstrate high dielectric polar moments. The rapid alternate frequencies produce sufficient mechanical agitation of the dipolar molecules, resulting in elevated temperatures at the interface of the film layers. Melting or fusion results. The most common materials demonstrating satisfactory loss tangents include polyvinyl chloride, polyurethane, polyamides, and similar plastic films or coatings on suitable woven or non-woven fabric substrates. The heating process is confined to the area confined by the surface of plano-parallel electrodes exerting, concurrently, compressive forces on the plastic load. As the plastic materials located between opposed sealing electrodes becomes heated, a plastic flow is created forcing some molten material outwards from between the electrodes' faces. The void formed by the extrudate is filled by one or both electrodes sinking into the faces of the films. A press supplies the compressive forces on opposing electrodes and permits the electrode faces to be urged together during the extruding phase. This phenomenon, commonly called press "follow through," is required to successfully co-mingle the molten interstitial faces between films to be welded, and to limit the possibility of arcing or breakdown due to the voids created when the extrudate leaves the area defined by the electrode sealing surfaces. The inertia and speed of travel of the press and electrode system must be such that controlled compressive forces of substantial magnitude are maintained at all times that the plastic load in the sealing area is liquid. This eliminates the practicability of utilizing a press which derives its compressive force by use of manual pressure. The shear forces vary during the heating cycle dependent upon the temperature created by the dielectric field. Accordingly, the press "follow through" must proceed at a sufficiently high speed to accommodate various heating rates. Coincidental with the press motion upon liquifaction of the load is the need to maintain parallelism between the electrode faces. Arcing or breakdown of the load tends to occur when a portion of an electrode face sinks inordinately into the load. The voltage stress in the work load increases in those areas in which the plastic load is thinned out excessively, resulting in the increased probability of the occurrence of arcs.
The application of the Radio Frequency voltage to the work must follow the point in time at which the work is under compression by the electode faces. Premature application of the Radio Frequency voltage results in arcing at those points along the electrode face at which air gaps appear between the sealing surface of the electrode and the mating surface of the plastic load.